1. Field of the Invention
This invention relates to apparatus for the storage of fluids, and more particularly to pressurized storage tank systems with automatic shut-down of fluid flow upon the detection of a leak in the system.
2. Description of the Related Art
Bulk storage tanks are often used to store toxic chemicals, such as chlorine and sulfur dioxide. When a manufacturing plant or water treatment facility requires a ready supply of chlorine, for example, a bulk storage tank may be installed at a convenient location. Supply lines are used to connect the bulk storage tank with the facility. The storage tank may, for example, have a capacity of from 150 pounds to 90 tons of chlorine. A pressurized storage tank typically comprises a large cylindrical pressure vessel positioned longitudinally on its side. On the top surface of the tank, the pressure vessel extends upward in a cylindrical extension called the tank neck. Outlet angle valves extend upwardly from the neck and are used to regulate the flow of contents from the tank. A hand wheel is used to continuously adjust the flow by turning the outlet angle valve stem. The outlet valves end in a pipe fitting extending at an angle of 90.degree. from vertical for connection to the supply lines that carry the chlorine gas and liquid to the facility. A cylinder with a hinged lid may be attached to the neck, covering the outlet valves. Such a cylinder is referred to as a manway.
Pressurized storage tanks themselves rarely leak. Rather, the outlet valves, associated fittings, or supply lines may fail or become damaged and leak. Leaks are dangerous because, as noted, pressurized storage tanks are frequently used for storing toxic chemicals that can cause severe health and safety problems and damage the environment. Chlorine gas is extremely dangerous because it displaces air, and anyone breathing it can be quickly overcome by the gas and will suffocate. Recently enacted government and industry standards call for stationary pressurized storage tanks to be housed in a building structure that includes a "scrubber" and a leak detector. The scrubber is for neutralizing any chlorine gas that escapes from the tank, its valves, or fittings located within the structure. The leak detector is for detecting the presence of the chlorine outside of the tank but within the structure. The scrubber housed in the structure contains a substance known as a "caustic" that mixes with chlorine gas to transform it into, essentially, liquid chlorine bleach. When a leak is detected, the scrubber is energized, reacting the chlorine gas with the caustic, resulting in a relatively safe liquid bleach. The bleach can then be safely removed. The scrubber thereby removes the dangerous gas from within the structure, and then repairs on the leak may proceed.
A leak may occur at any time of day and may go undetected for some time, depending on the location of the tank, the maintenance schedule, and the rate of use. Therefore, the scrubber systems must contain sufficient caustic to be capable of neutralizing the entire contents of a conventional storage tank. Depending on how quickly the contents must be neutralized, and depending on the flow pressures involved, a 90 ton capacity pressurized storage tank may require a scrubber system with more than 90 tons (18,000 gallons) of caustic. As noted, tanks themselves rarely leak. Typically, the valves or supply lines fail. If one could be certain of discovering and shutting down the flow of the chlorine before all 90 tons escaped from the tank, it would not be necessary to provide for neutralizing the entire contents of a pressurized storage tank. Furthermore, there would be much less damage to the environment if one could be certain of stopping a leak shortly after it occurs. There would be much less danger to maintenance crews in coming upon the scene of a leak. Any leaks that did occur would be far safer and easier to clean up. Finally, the scrubber systems involved could be constructed with much lower caustic capacity and therefore at much less cost.
One automatic tank shut-down system for railroad tank cars has been developed by The Clorox Company, and is discussed in The Handbook of Chlorination (1988), by George Clifford White, at pages 626-627. This shut-down system makes use of a motor operated valve actuator. The actuator fits over the tank car outlet valve and comprises a D.C. motor designed to exert a 50 foot-pounds torque on the outlet valve stem. The actuator torque has a self-limiter, which prevents the actuator from shearing off the valve stem or otherwise damaging the valve. Thus, the shut-down system stops the flow of tank contents by turning the hand wheel of the valve. After a shut-down, the tank car outlet valve must be opened manually by turning the hand wheel. A self-contained D.C. power source protects against a disruption in operation due to power failure. When a leak is detected, the D.C. motor must be activated to apply the necessary torque to the valve stem and shut down the fluid flow.
Although the Clorox tank car shut-down system is suitable for its intended application, circumstances may arise under which the system is not ideal. For example, the shut-down system actually operates on the outlet valve of the tank. As noted, pressurized storage tanks themselves rarely leak, but their valves or associated fittings may fail. The Clorox automatic shut-down system will have no effect on a leak if the source of the leak is the tank outlet valve. Furthermore, no shut-down will occur if there is damage to the D.C. motor or if parts of the motor become worn out. If any such damage occurs, the tank valve may likely be in the open state, and will have to be closed manually. Thus, the risk of leakage is not greatly reduced. Even if the shut-down system operates as intended, and the valve is not the source of the leak, the valve must still be manually opened once repairs have been completed. Finally, because the actuator fits over the tank car outlet valve, the shut-down system is vulnerable to exterior damage, including corrosion, vandalism, and tank car derailment.
Thus, there is a need for a pressurized storage tank with automatic shut-down of the tank flow upon detection of a leak. The shut-down should occur quickly after leak detection, for minimizing the required capacity of scrubber systems. The shut-down system should not be vulnerable to external damage or to power or control failure. Finally, the system should be capable of convenient resumption of flow after shut-down.